In the nuclear power plant, the reliability in controlling the 6.6 kV switchgear of the transformer is essential to the safe operation of the 6.6 kV auxiliary power system, the 380V auxiliary power system and the nuclear power units, and also to the safe operation of the nuclear island. In the prior art, the 6.6 kV switchgear of the transformer is controlled by the relay control device in the switchgear, including the intermediate relay, the protective current relay and the voltage relay, and the switching-through intermediate relay mounted on the relay rack, the control switch, the measurement apparatuses and the cable mounted in the control room, to realize the controlling, interlock, measurement and monitoring of the switch. It is complicated in connection, low in reliability, difficult in maintenance and low in efficiency, while many operators are needed for maintenance such that the cost of operation is increased.